The blood coagulation factors are distributed in plasma, with various types of factors from 1st coagulation factor to 13th coagulation factor working in cascades fashion to result in blood coagulation. The mechanism wherein individual blood coagulation factors participate in blood coagulation is shown in FIG. 1.
As shown in FIG. 1, blood coagulation is accomplished by a series of reactions which are very sophisticated and complicated. In general, inactivated precursors are activated by specific active blood coagulation factors (indicated by “a” attached to the end of coagulation factor). Then, next blood coagulation factors are activated. Most of those activated blood coagulation factors are enzymes of serine protease family. They adhere on the surface of activated platelet at wound site and activate blood coagulation factors stepwise and finally produce fibrin clot, leading to hemostasis.
Thrombin is a multi-functional coagulation factor that is involved in the final stages of the coagulation cascades. Prothrombin, the precursor of thrombin, is activated by prothrombinase complex composed of factor Va, factor Xa, Ca++, and phospholipids (PL) to yield thrombin, which converts fibrinogen to fibrin. The generated fibrins cover the aggregated platelet to induce blood coagulation. Finally, fibrins are cross-linked by factor XIIIa to produce stable fibrin clot.
To produce prothrombinase complex, factor X has to be activated to factor Xa, which is mainly mediated by Xase complex. Factor VIIIa, factor IXa, Ca++ and phospholipids (PL) generated via intrinsic pathway or factor VIIa, tissue factor (TF) and Ca++ generated via extrinsic pathway work as Xase complex.
Thrombin also activates factor V and factor VIII. When thrombin is over-produced, blood vessel itself may be clogged. To avoid the clogging, thrombin triggers blood coagulation inhibition action. That is, thrombin is binding to thrombomodulin to activate protein C. The activated protein C (APC), complexed with protein S, inactivates the factor Va and factor VIIIa.
In fact, factor Xa itself is a serine protease, and involved in the complicated blood coagulation process. Factor Xa, as an essential member of prothrombinase complex, is acting as a catalyst for conversion of prothrombin to thrombin. Thrombin converts fibrinogen into fibrin monomers and the fibrin monomers thus generated is involved in the generation and the stabilization of thrombus. Thus, over or inappropriate production of thrombin might result in thromboembolism. Therefore, the inhibition of thrombin itself or thrombin generation may result in the reduction of fibrin production involved in thrombus formation, leading to the prevention of thromboembolism.
In brief, the inhibition of factor Xa results in the inhibition of thrombin production, by which thromboembolism can be prevented or alleviated. The compound represented by formula I in the present invention and the pharmaceutically acceptable salt thereof can inhibit factor Xa, which eventually, according to the above logic, leads to the prevention of thromboembolic diseases (MI, stroke, PE, etc.).
Among compounds known as factor Xa inhibitors, antistasin (ATS) and tick anti-coagulant peptide (TAP) are representative protein inhibitors. ATS (composed of 119 amino acids) is a natural peptide isolated from leech, having Ki value of 0.05 nM against factor Xa. TAP is also a peptide isolated from tick which is composed of 60 amino acids and has Ki value of 0.5 nM against factor Xa. However, these inhibitors are in limited clinical use; only heparin or its sulfated polysaccharide analogues are in clinical use with some limitation.
A low molecular compound was developed as a blood coagulation inhibitor, particularly factor Xa inhibitor which is described in WO9529189. In the meantime, WO9933800 describes factor Xa inhibitor having indole moieties. In addition, diverse factor Xa inhibitors are discovered and in the process of development. For example, heterocyclic compound having nitrogen atom (WO2004058743), imidazole derivatives (WO2004050636), pyrazole derivatives (WO2004056815), indole-2-carboxamide derivatives (WO2003044014), oxybenzamide derivatives (WO2002051831), guanidine/amidine derivatives (WO2002046159), amino-bicyclic pyrazinone/pyridinone derivatives (WO2004002405), etc.
To be clinically useful, as FXa inhibitors, these molecules should have high antithrombotic effect, high stability in both plasma and liver, proper selectivity to other related serine proteases (thrombin, trypsin, cathepsin G, etc), low toxicity, and satisfactory bioavailability.
The most advanced compound having oxazolidinone, similar to that of the present invention, is Rivaroxaban (formula A), which is now at phase III clinical evaluation. Some oxazolidinone derivatives represented by formula 2 are described in WO 01/47917. However, some of these compounds are reported to have limited solubility; a specific example of the problem is Rivaroxaban. The solubility of Rivaroxaban is only 8 mg/L. The poor solubility may give rise to a lot of practical limitations including variability, and slow dissolution. These problems may be circumvented by introducing a highly soluble moiety.

WO 2004/83174 describes the use of pyrazole derivatives including Apixaban. Some of these inhibitors are cyclic amidine and sulfonyl amidine derivatives represented by formula C.
However, there is no precedent similar to the present invention which describes the specific introduction of cyclic amidoxime or cyclic amidrazone in oxazolidinone scaffold as factor Xa inhibitors. In fact, little is known about the substantiation of cyclic amidoxime or cyclic amidrazone in drug design.

The major trend of recent studies on FXa and thrombin inhibitor is the implementation of amidine functions. The amidine function, so called P1 group, is designed to bind Asp189 located on the bottom of S1 pocket. Both FXa and thrombin recognize arginine residue in natural substrate as the P1 site. Amidine group (including guanidine derivatives) replacing guanidine in arginine is highly hydrophilic. Thus, inhibitors with amidine function are generally not well absorbed, and even if they are absorbed, they are cleared too fast due to the intrinsic high polarity of the amidine (Drugs of the Future, 1999, 24(7), 771).
Amidine itself has a strongly basic character (PKa: approximately 12.5). Due to a formal positive charge at physiological condition, the amidine inhibitor generally shows poor absorption. Therefore, it needs to be changed to less basic alternatives. Representative examples of these are pyridine derivative, amidrazone, cyclic amine, alkylamine derivative, aminobenzisoxazole, etc (U.S. Pat. No. 6,958,356). There are also fundamentally different approaches to circumvent these problems, including amidoximes. Amidoxime is easily synthesized by adding hydroxyl group to amidine structure, which is a prodrug based on that weak N—O bond is easily reduced to amidine in vivo. This approach takes advantage of that PKa of amidoxime is remarkably lower (8-9) than that of amidine. Ximelagatran is another example of the same type of prodrug. This trend is seen not only in the study of FXa inhibitor but also in the study of thrombin inhibitor. However, most of these attempts turned out to be not as good as expected. As a third class of attempt, neutral P1 groups are introduced. Unlike other class of drugs, FXa and thrombin inhibitors tend to show good efficacy when they are at high concentrations in blood. Moreover, the concentration of free drug, unbound to serum proteins in blood, is very important. In the case of neutral P1 group inhibitor, protein binding tends to be high, resulting in poorer efficacy than expected.
To overcome these problems, the present inventors introduced relatively polar group at other positions than P1 site, with a neutral group fixed at P1 site. Parallel to the logic, some other important factors to improve pharmaceutical effect are included, which are as follows: 1) Substantial improvement of water-solubility; 2) Low plasma protein binding; when free concentration of the drug is high, the efficacy in PT assay increased likewise even though FXa binding affinity is somewhat compromised.
The site selected for the introduction of polar group in this invention is P4 sub-site of inhibitor, and the logic behind the selection is as follows. S4 site of FXa has U-shaped binding site, surrounded by three faces with Tyr99, Phe174, and Trp215. The binding site is composed of only aromatic amino acid side chains, which is different from thrombin surrounded by Leu99, Ile174, and Trp215. The difference is exploited in the drug design.
S4 pocket of FXa is good at interaction with cationic residue, which is generally called “π-cation interaction”. In fact, some inhibitors are designed and synthesized to have positive charge in P4 sites. In this invention, cyclic amidoxime or amidrazone is introduced in P4 site, in order to improve water-solubility and increase drug effect by reducing protein binding, as mentioned above. The reason of selection cyclic form is that the inventors believed that absorption could be improved by reducing number of NH bonds which generally shows negative effect on absorption. According to recent studies, it is more advantageous for a drug to have less hydrogen bond donor (HBD) than hydrogen bond acceptor (HBA). According to Lipinski's Rule, up to 10 HBAs are possibly accepted but HBD is limited only up to 5 (Adv. Drug Delivery Rev., 2001, 46, 3-26) and particularly in the case of new drug the number of average HBDs is approximately 2, suggesting that HBD is more strictly restricted (J. Med. Chem. 2004, 47, 6338-48). Amidoxime or amidrazone group itself has basic character, which enables easier separation-purification-storage as a salt form, and as a result, water-solubility is expected to be increased.
In short, we introduced amidoxime or amidrazone at P4 site. To reduce number of HBD, we used cyclic form of the function to make more drug-like inhibitors.
The compounds of formula I of the present invention were in fact confirmed to have the aforementioned advantages. Water-solubility and protein binding level are presented, together with 2×PT value and Ki.